Many vehicles produced by automotive manufacturers include anti-lock braking control systems (ABS) typically designed to detect and prevent the occurrence of a wheel lock-up condition during commanded braking of the vehicle. The wheel lock-up condition is prevented by cycling or modulation of the hydraulic pressure to the wheel brake(s) in a known manner by, for example, providing a series of releases and applies of the hydraulic brake pressure.
Many ABS systems use solenoid valves that are controllably opened and closed to increase (apply) and decrease (release) the hydraulic brake pressure to the vehicle brakes. In one known method, an apply solenoid is used to control application of brake pressure from a high pressure hydraulic source, such as a fluid pump, to the wheel brakes and a release solenoid is used to control the release of brake pressure from the fluid brakes to a low pressure brake fluid reservoir or accumulator.
In some systems, during the ABS event, the substantially steady state running current or speed of the hydraulic pump motor is monitored to determine master cylinder pressure, which is used as a control input for the ABS system in a known manner. One challenge to this approach is that different pumps and pump motors, even of the same design, have variations from part to part that affect the relationship between pump running current or speed and pressure output from the pump. For example, tolerance variations in wire used to wind the motor coils can affect the impedance of a motor and therefore also affect the relationship between motor current and output load. Similarly, variations in mechanical parts can affect the internal friction of the motor and of the pump and can also affect the relationship between motor current and output load (i.e., hydraulic pressure at the pump outlet).